In some belt-drive continuously variable transmissions (belt-drive CVTs), a ratio-changing actuator or a shift actuator, such as a step motor, is brought into a control position (the number of angular steps) corresponding to a target transmission gear ratio (or a target pulley ratio), thereby producing a differential pressure between primary and secondary pulley pressures in response to the target transmission gear ratio. The V-groove widths of primary and secondary variable-width pulleys, whose V grooves are aligned with each other, are varied by way of the differential pressure between the primary and secondary pulley pressures. Consequently, the target transmission gear ratio is achieved. Generally, in determining the number of angular steps (denoted by “Step”) of the step motor, a theoretical transmission gear ratio, denoted by Ip and corresponding to the engine/vehicle operating conditions, is taken or regarded as a target transmission gear ratio I(0). The number of angular steps of the step motor (the shift actuator) is calculated or retrieved based on the target transmission ratio I(0) from a preprogrammed angular steps Step versus transmission ratio ip characteristic map shown in FIG. 14. The line pressure, from which primary and secondary pulley pressures originate, uses a medium consisting of working fluid (transmission oil) coming from an oil pump, which is driven by the engine. Thus, the pressure level of line pressure greatly affects the engine's fuel economy or the fuel consumption rate of the vehicle. For the reasons discussed above, it is customary to design the CVT, such that the line pressure is adjusted to a bare minimum value. However, during execution of line pressure control, there is a possibility that the line pressure will become deficient, for example, due to variations between hardware units or each hardware's individual operating characteristics, and thus the line pressure level will become less than the primary or secondary pulley pressure. Assuming that the line pressure becomes deficient, the actual Step-ip characteristic tends to deviate from the preprogrammed Step-ip characteristic shown in FIG. 14. For instance, the preprogrammed Step-ip characteristic of FIG. 14 is displaced as indicated by the solid line in FIG. 11, where the Step-ip characteristic not displaced is indicated by the broken line in FIG. 11. In the presence of the deficient line pressure produced, for the same target transmission gear ratio I(0), more angular steps are required. In particular, the target transmission gear ratio I(0) cannot be achieved, unless an instruction (a control command) is given to the step motor to handle extra angular steps a. Owing to such a deficient line pressure level, there is an increased tendency for the achievement of the target transmission gear ratio I(0) to be delayed. In the case that the highest transmission ratio is required, it will not be achieved. Suppose that, in a conventionally adopted scheme, the primary pulley pressure is controlled to control the differential pressure between the primary and secondary pulley pressures. The above-described case where the highest transmission ratio is not achieved is described below by referring to FIG. 15 regarding this conventional scheme. In FIG. 15, the horizontal axis indicates the valve stroke (i.e., the valve opening) of a shift control valve controlled by means of the step motor, while the vertical axis indicates the hydraulic pressure. When the line pressure is sufficiently high, the primary pulley pressure originating from the line pressure reaches the required primary pulley pressure corresponding to the target transmission gear ratio Ip (=the highest transmission ratio) at a stroke amount L1 as indicated by the solid line in FIG. 15. Thus, the highest transmission ratio can be achieved. However, when the line pressure is lower than the required primary pulley pressure corresponding to the target transmission gear ratio Ip (=the highest transmission ratio), the primary pulley pressure originating from the line pressure does not reach the required primary pulley pressure indicated by the broken line in FIG. 15. Hence, the highest transmission ratio is not reached. For the reasons discussed above, the earlier line pressure control apparatus corrects the line pressure based on the deviation |Astep-Bstep| between the number of angular steps Astep of the stepping motor corresponding to the target transmission gear ratio I(0) and the number of angular steps Bstep calculated or estimated based on the actual transmission gear ratio ip. In the earlier line pressure control apparatus, the line pressure is corrected just before a particular state, where the highest transmission ratio is not reached, takes place, in other words, immediately after the line pressure becomes deficient or excessive. As set forth above, the earlier line pressure control apparatus initiates or executes line pressure correction (or line pressure compensation), in the case that the line pressure becomes deficient, as well as in the case that the line pressure becomes excessive, thus avoiding the problem that the approach of actual transmission gear ratio ip to target transmission gear ratio I(0) is delayed. One such line pressure control apparatus has been disclosed in Japanese Patent Provisional Publication No. 2004-100737 (hereinafter is referred to as “JP2004-100737”).